1. Field
Embodiments of the present invention relate to laser systems and, in particular, to tunable external cavity diode lasers systems.
2. Discussion of Related Art
Tunable lasers are deployed in such applications as telecommunication network test systems, spectroscopy research , and sensing for process control. They are also becoming recognized as essential components in the rapidly growing field of wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d) for optical communication systems. There are various well-known or proprietary designs for tunable lasers and each is usually tailored for a particular use. The specific design depends on many factors, including the light beam (e.g., power, wavelength, tunability), operating environment (e.g., temperature), and practical considerations (e.g., size, cost).
FIG. 1 shows a typical tunable external cavity laser 100. The laser 100 includes a gain medium 102, which has one facet 104 anti-reflective (AR) coated and another facet 106 coated with a highly reflective material. The gain medium 102 is typically a diode laser. The light output from the facet 104 is collimated by a lens 108 onto a diffraction grating 110, which diffracts the light towards a mirror 112. The mirror 112 reflects a particular wavelength back to the grating 110 and the gain medium 102. The facet 106 and the grating 110 form a cavity. The output of the laser 100 is the light beam 114. To tune the laser to another wavelength, the mirror 112 is adjusted accordingly. There are other well-known tunable external cavity laser designs, such as a well-known Littrow external cavity laser, which has one or more frequency-selective components positioned in the cavity for tuning.
To ensure proper operation of any tunable laser including the tunable laser 100, many of the parameters (e.g., power, wavelength, temperature) are controlled and monitored by servo control loops. It is common for each parameter to have its own separate control loop for setting, updating, and sampling laser parameters such as power, channel, and temperature. Separate loops can be problematic because each control loop operates asynchronously and the noise generated by sampling and/or updating from each loop could interfere with sampling of one or more of the other loops (cross talk). The noise may present itself as intermittent noise sources, which adds to the overall system noise and potentially degrades system performance. Typically, there are many control loops in a tunable laser system, thus many potential opportunities for cross talk. Noise also can come from a non-ideal ground plane, a power supply load change, unwanted coupling from digital I/O lines, or other sources.
Noise is commonly minimized by a combination of amplifying the signal of interest and filtering out the noise. However, as tunable lasers are moving towards smaller form factors noise compensation using amplifier circuits, filtering circuits, and other signal-to-noise ratio (SNR) increasing circuitry may not be appropriate.